linux-kernel - Re: [PATCH 1/2] f2fs: improve readahead for POSIX_FADV

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Message-ID: <aSCpzRW8mUhNnjHB@google.com>
Date: Fri, 21 Nov 2025 18:05:01 +0000
From: Jaegeuk Kim <jaegeuk@...nel.org>
To: Christoph Hellwig <hch@...radead.org>
Cc: linux-kernel@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
	Matthew Wilcox <willy@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/2] f2fs: improve readahead for POSIX_FADV_WILLNEED

On 11/20, Christoph Hellwig wrote:
> On Fri, Nov 21, 2025 at 01:42:01AM +0000, Jaegeuk Kim wrote:
> > This patch boosts readahead for POSIX_FADV_WILLNEED.
> 
> How?  That's not a good changelog.
> 
> Also open coding the read-ahead logic is not a good idea.  The only
> f2fs-specific bits are the compression check, and the extent precaching,
> but you surely should be able to share a read-ahead helper with common
> code instead of duplicating the logic.

Ok, let me try to write up and post a generic version of the changes.

> 
> > 
> > Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
> > ---
> >  fs/f2fs/data.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++++
> >  fs/f2fs/f2fs.h |  1 +
> >  fs/f2fs/file.c |  9 +++++---
> >  3 files changed, 68 insertions(+), 3 deletions(-)
> > 
> > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > index a0433c8a4d84..d95974d79fb3 100644
> > --- a/fs/f2fs/data.c
> > +++ b/fs/f2fs/data.c
> > @@ -2710,6 +2710,67 @@ static void f2fs_readahead(struct readahead_control *rac)
> >  	f2fs_mpage_readpages(inode, rac, NULL);
> >  }
> >  
> > +int f2fs_readahead_pages(struct file *file, loff_t offset, loff_t len)
> > +{
> > +	struct inode *inode = file_inode(file);
> > +	struct address_space *mapping = file->f_mapping;
> > +	pgoff_t start_index = offset >> PAGE_SHIFT;
> > +	loff_t endbyte = offset + len;
> > +	pgoff_t end_index;
> > +	unsigned long nrpages;
> > +	unsigned long ra_pages = (16 * 1024 * 1024) / PAGE_SIZE;
> > +	DEFINE_READAHEAD(ractl, NULL, &file->f_ra, mapping, start_index);
> > +
> > +	if (!S_ISREG(inode->i_mode))
> > +		return -EOPNOTSUPP;
> > +
> > +	/* Should be read only. */
> > +	if (!(file->f_mode & FMODE_READ))
> > +		return -EBADF;
> > +
> > +	/* Do not support compressed file for large folio. */
> > +	if (f2fs_compressed_file(inode))
> > +		return -EINVAL;
> > +
> > +	if (!mapping || len < 0)
> > +		return -EINVAL;
> > +
> > +	if (unlikely(!mapping->a_ops->read_folio && !mapping->a_ops->readahead))
> > +		return -EINVAL;
> > +
> > +	/* Load extent cache at the first readahead. */
> > +	f2fs_precache_extents(inode);
> > +
> > +	/*
> > +	 * Careful about overflows. Len == 0 means "as much as possible".  Use
> > +	 * unsigned math because signed overflows are undefined and UBSan
> > +	 * complains.
> > +	 */
> > +	if (!len || endbyte > i_size_read(inode) || endbyte < len)
> > +		endbyte = i_size_read(inode) - 1;
> > +	else
> > +		endbyte--;		/* inclusive */
> > +
> > +	/* First and last PARTIAL page! */
> > +	end_index = endbyte >> PAGE_SHIFT;
> > +
> > +	if (start_index > end_index)
> > +		return 0;
> > +
> > +	nrpages = end_index - start_index + 1;
> > +
> > +	while (nrpages) {
> > +		unsigned long this_chunk = min(nrpages, ra_pages);
> > +
> > +		ractl.ra->ra_pages = this_chunk;
> > +
> > +		page_cache_sync_ra(&ractl, this_chunk << 1);
> > +
> > +		nrpages -= this_chunk;
> > +	}
> > +	return 0;
> > +}
> > +
> >  int f2fs_encrypt_one_page(struct f2fs_io_info *fio)
> >  {
> >  	struct inode *inode = fio_inode(fio);
> > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > index 3340db04a7c2..934287cc5624 100644
> > --- a/fs/f2fs/f2fs.h
> > +++ b/fs/f2fs/f2fs.h
> > @@ -4047,6 +4047,7 @@ int f2fs_init_bio_entry_cache(void);
> >  void f2fs_destroy_bio_entry_cache(void);
> >  void f2fs_submit_read_bio(struct f2fs_sb_info *sbi, struct bio *bio,
> >  			  enum page_type type);
> > +int f2fs_readahead_pages(struct file *file, loff_t offset, loff_t len);
> >  int f2fs_init_write_merge_io(struct f2fs_sb_info *sbi);
> >  void f2fs_submit_merged_write(struct f2fs_sb_info *sbi, enum page_type type);
> >  void f2fs_submit_merged_write_cond(struct f2fs_sb_info *sbi,
> > diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> > index d7047ca6b98d..b6f71efd6d2a 100644
> > --- a/fs/f2fs/file.c
> > +++ b/fs/f2fs/file.c
> > @@ -5305,9 +5305,12 @@ static int f2fs_file_fadvise(struct file *filp, loff_t offset, loff_t len,
> >  		filp->f_mode &= ~FMODE_RANDOM;
> >  		spin_unlock(&filp->f_lock);
> >  		return 0;
> > -	} else if (advice == POSIX_FADV_WILLNEED && offset == 0) {
> > -		/* Load extent cache at the first readahead. */
> > -		f2fs_precache_extents(inode);
> > +	} else if (advice == POSIX_FADV_WILLNEED) {
> > +		if (offset == 0 && len == -1) {
> > +			f2fs_precache_extents(inode);
> > +			return 0;
> > +		}
> > +		return f2fs_readahead_pages(filp, offset, len);
> >  	}
> >  
> >  	err = generic_fadvise(filp, offset, len, advice);
> > -- 
> > 2.52.0.487.g5c8c507ade-goog
> > 
> > 
> ---end quoted text---